1. Field of the Present Invention
The present invention relates to electric toothbrushes, and more specifically to electromagnetic motor drives that operate at resonant frequency to drive a brush head.
2. Description of the Prior Art
Electric toothbrushes are generally well known. Also well known are the benefits of driving the toothbrush head with a mechanism operating at resonance. At high operating frequencies, driving the toothbrush head by a resonant mechanism maximizes the displacement of the brush head which optimizes the removal of plaque and stains from teeth. The resonant mechanism driving the brush also allows the brush head to move at these higher frequencies with more efficient power usage. Electric toothbrushes provide benefits not obtainable by manual brushing, including more complete and consistent cleaning action.
Numerous electric toothbrushes have been developed that employ spring-mass systems driven by electromagnetic motors operating a resonant mechanism to drive the brush head at resonant frequencies. Typically, the rotor, brush shaft and brush head constitute the mass, while a spring is used to urge the brush shaft to a rest position and assist in obtaining resonant frequency.
U.S. Pat. No. 2,917,758 to Held, et. al., describes an electrically controlled toothbrush having an elongated handle containing an electric motor and a shaft attached to the electric motor. The shaft extends through the handle and terminates in a removable brush head. The shaft is attached to the rotor of the electric motor. When an alternating current is applied to the stator coil, the rotor and shaft are subjected to a reciprocating torque which causes them to rotate in an oscillating manner about the longitudinal axis of the shaft at the frequency of the voltage applied. A spring is connected to the rotor to urge the rotor into a rest position. To maximize the amplitude of the oscillating motion of the shaft, the moment of inertia of the rotor and shaft and the characteristic properties of the spring are selected so that the natural mechanical resonant frequency of the rotor and shaft is close to the frequency of the power supply.
U.S. Pat. No. 3,535,726 to Sawyer describes an electric toothbrush having an elongated handle containing an electric motor and a shaft attached to the electric motor. The shaft extends from an end of the handle and terminates in a brush head. The motor works to actuate the brush head in a variety of sinusoidal motions. The shaft external to the handle acts as a cantilever resonating beam and has the same resonant frequency as the power supply. The shaft is attached to the motor such that the rotation of the motor excites the shaft in an eccentric manner, thus creating the sinusoidal movement of the brush head. The movement of the brush head generates sonic waves to clean the teeth of the user.
U.S. Pat. No. 3,538,359 to Barowski describes an electric toothbrush having a handle containing an electric motor and a shaft attached to the motor and extending from the handle. The shaft terminates in a brush head. Upon actuation by the electric motor, the shaft oscillates about its longitudinal axis. Springs are attached to the armature to urge the armature to a rest position, and also facilitate the oscillating motion of the brush head. No mention is made of using resonating frequencies in the Barowski patent.
U.S. Pat. No. 4,787,847 to Martin, et. al., describes an electric toothbrush having an elongated handle from which extends a piezoelectric transducer having a brush head attached to its distal end. The piezoelectric transducer shaft is excited by an AC signal generated by a circuit within the handle. The circuit supplies a specific frequency signal to the shaft. The specific frequency is calculated to produce the desired resonant frequency vibration of the shaft. The resonant frequency of the transducer shaft is determined by its geometry.
U.S. Pat. Nos. 5,189,751 and 5,263,218 to Giuliani, describe an electric toothbrush having a toothbrush shaft driven by an electromagnetic motor. The toothbrush shaft, upon actuation by the motor, pivots about a pivot point to vibrate in a planar motion (up and down or side to side). A spring is utilized near the pivot point to urge the shaft toward its neutral position. The '218 patent has similar specification and claims as the '751 patent, however, the '218 claims further include an energy recovery circuit for storing reactive energy to eliminate or reduce the energy lost during operation.
The brush head vibrates at a frequency decided by the particular mass and pivot arrangement. The brush head, brush shaft, and pivot member are selected to have a natural mode of resonance at a predetermined frequency. If vibration at a resonant frequency is desired, the operating frequency of the drive system is selected to be relatively close to the resonant frequency of the mechanical system (brush head, brush shaft, pivot member). The drive system can be de-tuned to vibrate at an off-resonant frequency from the mechanical system. Upon a certain load being placed on the mechanical system, the frequency of vibration becomes resonant. Further loading then moves the vibration out of resonance. The characteristics of the brush head movement changes with loading.
The aforementioned patents do not describe a system by which the brush head vibration is maintained at resonance throughout use to obtain the benefits of resonant vibration for cleaning teeth. The driving signals and spring-mass systems for each design are carefully selected to obtain resonant frequency at a given load. There remains a need for an electric toothbrush that maintains the vibration of the brush head at its resonant frequency under the variety of loads encountered during use.